A voltage-controlled oscillator (VCO) is one of important basic components of a phase-locked loop (PLL) circuit in a modern communication system. A performance of the modern communication system may greatly depend on the performance of the PLL circuit, and a performance of the PLL circuit may greatly depend on a performance of the VCO. Therefore, the performance of the modern communication system may depend on the performance of the VCO to a great extent.
With continuous advances in the integrated circuit technique, feature sizes of devices and metal wirings are continuously reduced. Consequently, the devices have a limited linearity and a limited linear range.
FIG. 1 schematically illustrates a block diagram of a conventional VCO 11. In the conventional VCO 11, an input voltage vin may be directly applied to an input of the VCO 11, and an output signal with an oscillation frequency f may be output from an output of the VCO 11.f=f0+k*vin tm (1)
wherein,
f is an oscillation frequency of the output signal when a voltage value of a control signal at a VCO input is vin;
f0 is the oscillation frequency of the output signal when the voltage value of the control signal at the VCO input is 0;
vin is the voltage value of the control signal as the VCO input; and
k is a control factor of the oscillation frequency of the output signal of the yea
FIG. 2 illustrates an ideal curve showing an input voltage vs. an oscillation frequency of the conventional VCO 11 as shown in FIG. 1. The linear range of the conventional VCO 11 is −vinmax≦vin≦+vinmax, wherein k is a constant only depending on electric characteristics of the VCO 11
However, the linear range of the VCO 11 is only an approximation, which ignores a high-order effect of the VCO 11, but not really an ideal linear range, Even in the linear range of −vinmax≦vin≦+vinmax, k is not really a constant, as shown in a dashed line in FIG. 3.
To improve the linearity and increase the linear range of the VCO, k should not be seen as a constant.